1. Field of the Invention
The present invention is broadly concerned with ultrasound beam characterization devices and phantoms which are used to test the accuracy of and to calibrate ultrasonic diagnostic equipment presently in use by many hospitals and doctors. More particularly, it is concerned with such devices which preferably form a part of an ultrasound phantom having ultrasound characteristics (i.e., the transmission pattern of ultrasonic waves directed therethrough) closely mimicking the transmission pattern of similar ultrasonic waves directed through one or more portions of the human body. Important ultrasound characteristics such as wave velocities, and attenuation and scattering coefficients of the phantom of the invention are very similar to those of human tissue. Devices in accordance with the invention can thus be constructed for permitting accurate beam characterization in the context of a tissue equivalent material, which greatly facilitates diagnostic techniques using ultrasound devices.
2. Description of the Prior Art
Although diagnostic ultrasound equipment has been in use for a number of years, a persistent problem has plagued both the designers and users of such equipment. Specifically, a truly stable and uniform ultrasound phantom has not been available for calibrating and checking the equipment on a regular basis. Similarly, a tissue-mimicking ultrasound phantom having necessary stability and uniformity qualities has not heretofore been produced. Ideally, such a tissue-mimicking phantom should have the same ranges of uniform velocities of sound, attenuation coefficients, and scattering coefficients as human tissue, although for calibration purposes exact tissue-mimicking characteristics are not absolutely necessary.
A number of attempts have been made in the past to provide an effective ultrasound phantom. One such attempt is described in an article entitled "Tissue Mimicking Materials For Ultrasound Phantoms", by Ernest L. Madsen et al., Med. Phys., 5 (5), Sept./Oct. 1978. In the phantoms described in this article, water-based pharmaceutical gels containing uniform distributions of graphite powder and known concentrations of alcohol are employed. One drawback in this type of ultrasound phantom stems from the fact that the graphite tends to settle out at temperatures over 90.degree. F., thus irreversably altering the ultrasound properties of the phantom. Moreover, many gels employed can be unstable under certain conditions, primarily due to bacterial attacks on the gel and ambient temperature variations, thus leading to degradation of the gel and consequent failure of the phantom. Finally, it is difficult to achieve and maintain a uniform dispersion of the graphite, and to incorporate zones therein for the mimicking of cysts or the like.
Another known ultrasound phantom produced by researchers at the University of Colorado employs a base of silicone polymer combined with mineral oil, polystyrene or glass beads embedded therein. Various other substances are being investigated as phantom materials, and these include soft plastics such as plastisols, or urethane polymers. However, the phantoms produced to date are generally deficient in one or more important respects. For example, it is very difficult to remove air bubbles from the material and achieve reproducibly uniform concentrations of scattering particles.
Another problem associated with ultrasound diagnostic equipment relates to the inability to properly determine the effective transducer beam profile and focal length. Presently, such determinations are made by scanning wire targets in a liquid filled phantom such as the AIUM standard 100 mm. test object, and while this does give a beam profile and focal length, these values may be very different when the transducer actually scans human tissue. Thus, while a particular transducer may be rated with a given focal length and beam profile, actual use thereof in a diagnostic test can give very different results. In this regard it will be understood that if a given transducer is out of focus with respect to scanning a particular internal organ (e.g., a liver), the ultrasound diagnostic results are correspondingly less than optimum; therefore, a transducer having a proper focal length for scanning the organ in question should be employed if possible.
In response to this latter problem, workers in the art have bought and maintained an array of differently rated transducers, and have attempted to achieve proper focal length for a given patient or condition being scanned on essentially a trial and error basis. This is a time consuming process, wherein diagnostic success often depends heavily upon skill of the operator.
Accordingly, there is a real need in the art for an ultrasound beam characterization device which can be used to give more accurate transducer beam profiles and focal lengths, particularly in the context of a tissue-mimicking ultrasound phantom.